The backlighting of liquid crystal displays for avionics is usually accomplished by light boxes containing fluorescent tubes. These displays produce luminosities of the order of 1000 Cd/m2 necessary for daytime vision, sometimes in bright sunlight. Furthermore, the illumination output by the light box must be able to be adjusted so as to greatly reduce its luminosity during night flights, for example to levels of the order of 0.1 Cd/m2.
FIGS. 1a, 1b and 1c show various views of a light box of the prior art based on fluorescent tubes.
FIG. 1a shows a sectional view of a light box of the prior art, based on fluorescent tubes. The box in FIG. 1a, of rectangular shape, comprises a case 10 having a bottom 12 with an opening 14 that faces the bottom 12, for backlighting a liquid crystal display 16.
The bottom 12 of the case 10 has a reflector 20 and an array of fluorescent tubes 22 parallel to the bottom of the case and, across the opening 14, a light diffuser 24. The reflector 20 is of known shape so that the light rays r emitted by the fluorescent tubes are directed onto the light diffuser 24, uniformly illuminating the liquid crystal display.
The light box further includes a waveguide 30 in the form of a plate having two parallel faces 32, 34 and edges 36. The face 32 turned toward the opening 14 of the box includes a diffusing structure 38 for diffusing the light generated by a lateral fluorescent tube 40 illuminating one of the edges of the lightguide.
FIG. 1b shows a top view of the box of FIG. 1a, showing the arrangement of the lateral fluorescent tube 40 on the edge 36 of the lightguide 30 having the diffusing structure 38 on the face 32 turned toward the liquid crystal display.
FIG. 1c is a partial view of one edge of the lightguide, illuminated by the lateral fluorescent tube 40, showing the path of the light rays I diffused by the face 32 of the lightguide having the diffusing structure 38.
When the ambient light is strong, for example, during the day, the array of fluorescent tubes 22 is turned on, producing a high level of illumination for the liquid crystal display. At night, the illumination must be much lower. The array of fluorescent tubes is therefore turned off and only the single lateral fluorescent tube 40 is turned on, creating a low level of light diffused by the lightguide (or waveguide) 30 through the diffuser 24 toward the liquid crystal display 16.
Furthermore, the range of variation of the light intensity offered by the light box may be extended by controlling the power supplied to the tubes. For this purpose, the supply voltage for the fluorescent tubes is in the form of rectangular pulses having a frequency ranging from a few tens to a few hundred hertz. The power supplied to the tubes may be adjusted by varying the duty cycle of the rectangular pulses.
The luminosity performance of light boxes based on fluorescent tubes is satisfactory for illuminating liquid crystal displays intended in particular for avionics. However, such boxes have a large volume and require regular maintenance owing to the lifetime of the fluorescent tubes.
It is known today to use light-emitting diodes or LEDs instead of fluorescent tubes for producing light boxes. The progress made in light-emitting diodes allows light-emitting diode light boxes to be produced that are less bulky than fluorescent tube light boxes and at a lower cost, whilst approaching the efficiencies and luminous intensities obtained with fluorescent tubes.
FIG. 2a shows a device for illuminating a light guide 41 comprising light-emitting diodes.
The device of FIG. 2a comprises two rows Ra1 , Rb2 of LEDs Lnm wired to a printed circuit 42. Each row Ra1 , Ra2 has seven LEDs connected in series (n being the number of the row, 1 or 2, and m the number of the LED in each row, 1 to 7). The rows of LEDs are aligned, one after another, along one of the edges B1 of the lightguide 41 in the form of a thin plate, having two opposed main faces and four edges B1, B2, B3, B4 which delimit the plate.
As in the light box shown in FIG. 2a, the lightguide 41 has, on one of the faces 44, an optical structure 50 for diffusing the light propagating in the lightguide.
The lightguide 41 transmits the light generated by the rows Ra1, Ra2 of light-emitting diodes L11, . . . L17 and L21, . . . L27, applied to its edge B1, which, by propagating in the lightguide, is diffused by its optical structure 50 uniformly over the entire face 44 of the lightguide, as already described above.
Each row Ra1, Ra2 of light-emitting diodes is connected to an electric power supply S1, for row Ra1, and to electric power supply S2, for row Ra2, respectively.
FIG. 2b shows the circuit diagram for connecting the rows of diodes Ra1 and Ra2, of FIG. 2a, to their respective power supplies S1 and S2.
The light boxes of the prior art shown in FIGS. 2a and 2b having a light source based on light-emitting diodes have however the drawback, in the event of a diode or a power supply failing, of causing a loss of uniformity of the luminosity on the surface to be illuminated.
This is because the result of one of the power supplies S1, S2 failing or one of the diodes of a row of light-emitting diodes being turned off is a loss of illumination on a half-edge (B1) of the box's lightguide and a variation in the luminosity according to the illuminated portion of the display. A variation in the luminosity of the light box, and consequently of the liquid-crystal display that it illuminates, may impair the legibility of instruments on board aircraft. This may be very troublesome for the pilot.